The invention relates to the field of adhesive materials. More particularly, the present invention relates to embedded compressive lever machines for increasing the strength of adhesive bondlines by modifying the internal stresses.
Adhesives have long been used to bond elements together. Filler material is typically added to adhesives to form suitable adhesive layers for bonding adherents together. For example, film epoxy adhesives contain fillers of various kinds to control thixotrophy, regulate flow of the adhesive during the cure cycle and enhance mechanical properties of the adhesive. Typical fillers include aluminum powder, calcium carbonate, mica, colloidal silica fibers, and glass fibers. Hard brittle adhesive systems can be modified by the use of fillers to increase toughness and consequently increase the peel properties of the adhesive. In addition, glass spheres can be added to reduce density or control the bondline thickness. None of the fillers used to date intentionally modify the internal stresses in the adhesive.
There are many advantages to using stronger adhesives. Increasing the strength of adhesives will be beneficial to joint bondings. When used in an existing joint design, an increased strength adhesive would add reliability to the part. A stronger part would have a higher factor of safety. Consequently, the joint would be more reliable during overloading or during deficiencies in the joint design. To take full advantage of the increased strength, new joint designs could be made smaller, more efficient and lighter than existing joints. When the adhesive strength is greatly increased, entirely new areas of technology can be envisioned where adhesives replace other methods of joining technology.
Typical bonded structures are designed so that the structural adhesive will be under shear loading most of the time. Adhesives are typically stronger under shear stresses than in tensile or peel loading. An adhesively bonded structure is considered in shear when the applied load acts in the plane of the adhesive layer. These loads tend to produce sliding of the adherents and this sliding results in sliding or shearing of the adhesive under shear stresses. These shear stresses function differently than pure shear stresses that are typically experienced by uniform blocks of homogenous material. Interactions between the adhesive thickness, adherent thickness, yield strength, and bond geometry produce nonuniform stresses. When nonuniform stresses occurs, tensile stresses rather than shear stresses may actually dominate the failure mode of the adhesive joints. Consequently, the joints may fail at a lower load than expected when the adhesive cannot also support high unwanted tensile loads. It is desirable to increase the shear strength of bonding joints to support increase shear stresses as well as unwanted tensile loads.
Recently, micromechanical systems have been adapted to provide embedded machines to augment composites. One class of materials contains embedded micromachines directed to modifying the properties of a composite material. Another class of embedded MEMS machines are fluid filled machines directed to specific applications where fluid filled machines are used to increase the damping properties of composites. These embedded machines have not been adapted to increase the shear strength of bondlines of a bonding adhesive. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to increase the shear strength of a bonding adhesive.
Another object of the invention is to increase the shear strength of a bonding adhesive using a micromachine.
Yet another object of the invention is to increase the shear strength of a bonding adhesive using an embedded micromachine that increases pressure upon the bonding adhesive orthogonally to shear load stresses.
Yet another object of the invention is to increase the shear strength of a bonding adhesive using an embedded lever micromachine that increases the compressive forces and presses upon the bonding adhesive orthogonally to shear load stresses.
The invention is directed to lever micromachines that are embedded in an adhesive bondline. Small micromachine levers are an added component into the adhesive film before the adhesive is cured in a bondline. The embedded levers tend to rotate under shear loads and during rotation applied pressure on the adhesive interface. The force caused by the levers creates a biaxial load on the adhesive interface consisting of a combination of compressive and shear stresses. The compressive component eliminates the tensile stress typically present in the bondline consequently eliminating the peel stresses. The peel stresses that may otherwise cause the adhesive bondlines to fail prematurely are eliminated thereby increasing the bondlines strength. In addition, the strength increase is large because the bondlines are in compression and many adhesives are much stronger when in compression.
The embedded lever micromachines increases the strength of adhesive bondlines allowing joints to be stronger and safer. Preferably, substantially identical micromachine levers are embedded inline in an adhesive material film so that several levers rotate concurrently for uniform compression along the adhesive bondline. Each lever machine is designed to rotate under shear loading. The rotation causes the top and bottom faces of the micromachine levers to compress the adhesive film at the interface to the adherents. Consequently, the adhesive is simultaneously under the influence of shear and compressive forces. The adhesive under the compressive forces is capable of carrying higher loads with higher shear strength without the influence of tensile forces. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.